Commercially available laser ranging instruments are generally based on a heterodyne method which continuously measures a phase and simultaneously mixes in a smaller frequency range. Both the intensity of the light source (laser) and the sensitivity of the detector are modulated at high frequencies with a small frequency shift with respect to one another. This creates a low-frequency beat, the phase shift of which is proportional to the distance of the measurement object.
A substantial advantage in this method is the low time-resolution demands on the electronics in the low-frequency signal processing range. However, a measurement, which is continuous in time and generally takes longer than the pulse circulation time between transmitter and receiver, is required. Good optical isolation between transmission and reception path is therefore necessary.
Such a laser ranging instrument is known from e.g. DE 10235562 A1.
3D laser scanners combine the function of ranging with a mechanical scanner that provides the option of realizing ranging sequentially at defined emission angles. The method for determining the distance is generally a pulse circulation time measuring method. A short pulse laser generates a light pulse which is directed at the measurement point targeted by the scanner via the transmission path. The pulse scattered back from the measurement point is imaged on a fast detector via the reception path. The light run-time between pulse generation and reception and hence the distance is determined for the targeted measurement point.
These 3D laser scanners are distinguished in particular by virtue of the fact that the optical axis of the reception path corresponds to the transmission path within the scanner assembly (there is no parallax). The mechanical deflection unit (scanner) is only required once and hence only needs to be aligned once as well. There are only low demands on the optical isolation between transmission and reception path. Since these are pulse circulation time measuring methods, there are, on the other hand, high demands on the time resolution capability of the signal processing electronics.
By way of example, such laser scanners are described in U.S. Pat. No. 5,988,862 or in EP 01209441 B1.